Printer with in-track position error correction

ABSTRACT

A digital printing system having a linear printhead includes corrections for in-track position errors. A data processing system implements a method for determining an in-track position function which includes printing a test target including a plurality of alignment marks, automatically analyzing a captured image of the printed test target to determine a measured in-track position for each of the alignment marks, comparing the measured in-track positions for the alignment marks to reference in-track positions to determine measured in-track position errors, and determining an in-track position correction function responsive to the measured in-track position errors. The in-track position correction function specifies in-track position corrections to be applied as a function of cross-track position. A corrected digital image is determined by resampling an input digital image responsive to the in-track position correction function.

CROSS-REFERENCE TO RELATED APPLICATIONS

Reference is made to commonly assigned, U.S. patent application Ser. No.16/417,731, entitled: “Correcting cross-track errors in a linearprinthead”, by Kuo et al.; to commonly assigned, U.S. patent applicationSer. No. 16/417,763, entitled: “Printer with cross-track position errorcorrection”, by Kuo et al.; and to commonly assigned, U.S. patentapplication Ser. No. 16/564,235, entitled: “Correcting in-track errorsin a linear printhead”, by Kuo et al., each of which is incorporatedherein by reference.

FIELD OF THE INVENTION

This invention pertains to the field of digital printing, and moreparticularly to a method for calibrating a printer including a linearprinthead to compensate for in-track registration errors.

BACKGROUND OF THE INVENTION

Electrophotography is a useful process for printing images on a receiver(or “imaging substrate”), such as a piece or sheet of paper or anotherplanar medium (e.g., glass, fabric, metal, or other objects) as will bedescribed below. In this process, an electrostatic latent image isformed on a photoreceptor by uniformly charging the photoreceptor andthen discharging selected areas of the uniform charge to yield anelectrostatic charge pattern corresponding to the desired image (i.e., a“latent image”).

After the latent image is formed, charged toner particles are broughtinto the vicinity of the photoreceptor and are attracted to the latentimage to develop the latent image into a toner image. Note that thetoner image may not be visible to the naked eye depending on thecomposition of the toner particles (e.g., clear toner).

After the latent image is developed into a toner image on thephotoreceptor, a suitable receiver is brought into juxtaposition withthe toner image. A suitable electric field is applied to transfer thetoner particles of the toner image to the receiver to form the desiredprint image on the receiver. The imaging process is typically repeatedmany times with reusable photoreceptors.

The receiver is then removed from its operative association with thephotoreceptor and subjected to heat or pressure to permanently fix(i.e., “fuse”) the print image to the receiver. Plural print images(e.g., separation images of different colors) can be overlaid on thereceiver before fusing to form a multi-color print image on thereceiver.

In-track position errors in digital printing systems having linearprintheads can result in objectionable in-track alignment errors betweencolor channels. Therefore, there remains a need for a method tocharacterize and correct for the in-track position errors that can beimplemented without the need for complex and costly fixtures.

SUMMARY OF THE INVENTION

The present invention represents a digital printing system incorporatingin-track position corrections, including:

one or more printing subsystems, each printing subsystem including alinear printhead extending in a cross-track direction including an arrayof light sources for exposing a photosensitive medium;

a data processing system;

a digital memory for storing an in-track position correction function;and

a program memory communicatively connected to the data processing systemand storing instructions configured to cause the data processing systemto implement a method for determining an in-track position correctionfunction for at least one printing subsystem, wherein the methodincludes:

-   -   a) providing digital image data for a test target including a        plurality of alignment marks positioned at predefined        cross-track positions;    -   b) printing the test target using the digital printing system to        provide a printed test target;    -   c) using a digital image capture system to capture an image of        the printed test target;    -   d) automatically analyzing the captured image to determine a        measured in-track position for each of the alignment marks;    -   e) comparing the measured in-track positions for the alignment        marks to reference in-track positions to determine measured        in-track position errors;    -   f) determining the in-track position correction function        responsive to the measured in-track position errors, wherein the        in-track position correction function specifies in-track        position corrections to be applied as a function of cross-track        position; and    -   g) storing a representation of the in-track position correction        function in the digital memory;

wherein the digital printing system is adapted to print digital imagesusing a printing process that includes:

-   -   i) receiving digital image data for a digital image to be        printed by the digital imaging system, wherein the digital image        includes a plurality of image lines extending in the cross-track        direction;    -   ii) determining corrected image lines by resampling the digital        image data responsive to the stored representation of the        in-track position correction function; and    -   iii) printing the corrected image lines using the one or more        printing subsystems to provide a printed image with reduced        in-track position errors.

This invention has the advantage that in-track alignment errors arereduced in a digital printing system.

It has the additional advantage that the in-track position correctionfunction can be determined using a simple process that includes printingand scanning a test target including a plurality of alignment marks.

It has the further advantage that the in-track position corrections canbe non-linear to account for localized in-track alignment errorcharacteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational cross-section of an electrophotographic printersuitable for use with various embodiments;

FIG. 2 is an elevational cross-section of one printing subsystem of theelectrophotographic printer of FIG. 1;

FIG. 3 shows a conventional processing path for producing a printedimage using a pre-processing system couple to a print engine;

FIG. 4 shows a processing path including a print engine that is adaptedto produce printed images from image data supplied by a variety ofdifferent pre-processing systems;

FIG. 5 shows additional details for the resolution modificationprocessor and the halftone processor of FIG. 4;

FIG. 6A shows an exemplary printhead including three light source tiles,each including 15 light source chips;

FIG. 6B shows an exemplary light source chip including a linear array of384 LEDs;

FIG. 7 shows a flowchart of a process for determining a positioncorrection function in accordance with an exemplary embodiment;

FIG. 8 illustrates an exemplary test target that includes alignmentmarks useful for determining a position correction function;

FIGS. 9A-9B illustrate the determination of measured alignment markpositions from a combined image trace;

FIG. 10A illustrates an exemplary cross-track position error functiondetermined using the method of FIG. 6;

FIG. 10B illustrates an exemplary position correction functioncorresponding to the cross-track position error function of FIG. 10A;

FIG. 10C illustrates a position correction function representationcorresponding to the position correction function of FIG. 10B;

FIG. 11 shows an improved processing path including a print engine thatis adapted to produce printed images incorporating cross-track positioncorrections in accordance with an exemplary embodiment;

FIG. 12 shows additional details for the resolution/alignment processorand the halftone processor of FIG. 11;

FIG. 13 illustrates a flow chart for a resampling operation thatcombines the resolution modification operation and the positioncorrection operation of FIG. 12 in accordance with an exemplaryembodiment;

FIG. 14 shows a flowchart of a process for determining an in-trackposition correction function in accordance with an exemplary embodiment;

FIG. 15 illustrates an exemplary test target that includes in-trackalignment marks useful for determining an in-track position correctionfunction;

FIG. 16A illustrates an exemplary in-track position error functiondetermined using the method of FIG. 14;

FIG. 16B illustrates an exemplary in-track position correction functioncorresponding to the in-track position error function of FIG. 16A;

FIG. 16C illustrates an exemplary in-track position correction functionrepresentation corresponding to the in-track position correctionfunction of FIG. 16B;

FIG. 17 shows an improved processing path including a print engine thatis adapted to produce printed images incorporating in-track positioncorrections in accordance with an exemplary embodiment;

FIG. 18 shows additional details for the resolution/alignment processorand the halftone processor of FIG. 17;

FIG. 19 illustrates an exemplary in-track position correction operation;and

FIG. 20 is a high-level diagram showing the components of a system forprocessing images in accordance with the present invention.

It is to be understood that the attached drawings are for purposes ofillustrating the concepts of the invention and may not be to scale.Identical reference numerals have been used, where possible, todesignate identical features that are common to the figures.

DETAILED DESCRIPTION OF THE INVENTION

The invention is inclusive of combinations of the embodiments describedherein. References to “a particular embodiment” and the like refer tofeatures that are present in at least one embodiment of the invention.Separate references to “an embodiment” or “particular embodiments” orthe like do not necessarily refer to the same embodiment or embodiments;however, such embodiments are not mutually exclusive, unless soindicated, or as are readily apparent to one of skill in the art. Theuse of singular or plural in referring to the “method” or “methods” andthe like is not limiting. It should be noted that, unless otherwiseexplicitly noted or required by context, the word “or” is used in thisdisclosure in a non-exclusive sense.

As used herein, the terms “parallel” and “perpendicular” have atolerance of ±10°.

As used herein, “sheet” is a discrete piece of media, such as receivermedia for an electrophotographic printer (described below). Sheets havea length and a width. Sheets are folded along fold axes (e.g.,positioned in the center of the sheet in the length dimension, andextending the full width of the sheet). The folded sheet contains two“leaves,” each leaf being that portion of the sheet on one side of thefold axis. The two sides of each leaf are referred to as “pages.” “Face”refers to one side of the sheet, whether before or after folding.

As used herein, “toner particles” are particles of one or morematerial(s) that are transferred by an electrophotographic (EP) printerto a receiver to produce a desired effect or structure (e.g., a printimage, texture, pattern, or coating) on the receiver. Toner particlescan be ground from larger solids, or chemically prepared (e.g.,precipitated from a solution of a pigment and a dispersant using anorganic solvent), as is known in the art. Toner particles can have arange of diameters (e.g., less than 8 μm, on the order of 10-15 μm, upto approximately 30 μm, or larger), where “diameter” preferably refersto the volume-weighted median diameter, as determined by a device suchas a Coulter Multisizer. When practicing this invention, it ispreferable to use larger toner particles (i.e., those having diametersof at least 20 μm) in order to obtain the desirable toner stack heightsthat would enable macroscopic toner relief structures to be formed.

“Toner” refers to a material or mixture that contains toner particles,and that can be used to form an image, pattern, or coating whendeposited on an imaging member including a photoreceptor, aphotoconductor, or an electrostatically-charged or magnetic surface.Toner can be transferred from the imaging member to a receiver. Toner isalso referred to in the art as marking particles, dry ink, or developer,but note that herein “developer” is used differently, as describedbelow. Toner can be a dry mixture of particles or a suspension ofparticles in a liquid toner base.

As mentioned already, toner includes toner particles; it can alsoinclude other types of particles. The particles in toner can be ofvarious types and have various properties. Such properties can includeabsorption of incident electromagnetic radiation (e.g., particlescontaining colorants such as dyes or pigments), absorption of moistureor gasses (e.g., desiccants or getters), suppression of bacterial growth(e.g., biocides, particularly useful in liquid-toner systems), adhesionto the receiver (e.g., binders), electrical conductivity or low magneticreluctance (e.g., metal particles), electrical resistivity, texture,gloss, magnetic remanence, florescence, resistance to etchants, andother properties of additives known in the art.

In single-component or mono-component development systems, “developer”refers to toner alone. In these systems, none, some, or all of theparticles in the toner can themselves be magnetic. However, developer ina mono-component system does not include magnetic carrier particles. Indual-component, two-component, or multi-component development systems,“developer” refers to a mixture including toner particles and magneticcarrier particles, which can be electrically-conductive ornon-conductive. Toner particles can be magnetic or non-magnetic. Thecarrier particles can be larger than the toner particles (e.g., 15-20 μmor 20-300 μm in diameter). A magnetic field is used to move thedeveloper in these systems by exerting a force on the magnetic carrierparticles. The developer is moved into proximity with an imaging memberor transfer member by the magnetic field, and the toner or tonerparticles in the developer are transferred from the developer to themember by an electric field, as will be described further below. Themagnetic carrier particles are not intentionally deposited on the memberby action of the electric field; only the toner is intentionallydeposited. However, magnetic carrier particles, and other particles inthe toner or developer, can be unintentionally transferred to an imagingmember. Developer can include other additives known in the art, such asthose listed above for toner. Toner and carrier particles can besubstantially spherical or non-spherical.

The electrophotographic process can be embodied in devices includingprinters, copiers, scanners, and facsimiles, and analog or digitaldevices, all of which are referred to herein as “printers.” Variousembodiments described herein are useful with electrostatographicprinters such as electrophotographic printers that employ tonerdeveloped on an electrophotographic receiver, and ionographic printersand copiers that do not rely upon an electrophotographic receiver.Electrophotography and ionography are types of electrostatography(printing using electrostatic fields), which is a subset ofelectrography (printing using electric fields). The present inventioncan be practiced using any type of electrographic printing system,including electrophotographic and ionographic printers.

A digital reproduction printing system (“printer”) typically includes adigital front-end processor (DFE), a print engine (also referred to inthe art as a “marking engine”) for applying toner to the receiver, andone or more post-printing finishing system(s) (e.g., a UV coatingsystem, a glosser system, or a laminator system). A printer canreproduce pleasing black-and-white or color images onto a receiver. Aprinter can also produce selected patterns of toner on a receiver, whichpatterns (e.g., surface textures) do not correspond directly to avisible image.

In an embodiment of an electrophotographic modular printing machineuseful with various embodiments (e.g., the NEXPRESS SX 3900 printermanufactured by Eastman Kodak Company of Rochester, N.Y.) color-tonerprint images are made in a plurality of color imaging modules arrangedin tandem, and the print images are successively electrostaticallytransferred to a receiver adhered to a transport web moving through themodules. Colored toners include colorants, (e.g., dyes or pigments)which absorb specific wavelengths of visible light. Commercial machinesof this type typically employ intermediate transfer members in therespective modules for transferring visible images from thephotoreceptor and transferring print images to the receiver. In otherelectrophotographic printers, each visible image is directly transferredto a receiver to form the corresponding print image.

Electrophotographic printers having the capability to also deposit cleartoner using an additional imaging module are also known. The provisionof a clear-toner overcoat to a color print is desirable for providingfeatures such as protecting the print from fingerprints, reducingcertain visual artifacts or providing desired texture or surface finishcharacteristics. Clear toner uses particles that are similar to thetoner particles of the color development stations but without coloredmaterial (e.g., dye or pigment) incorporated into the toner particles.However, a clear-toner overcoat can add cost and reduce color gamut ofthe print; thus, it is desirable to provide for operator/user selectionto determine whether or not a clear-toner overcoat will be applied tothe entire print. A uniform layer of clear toner can be provided. Alayer that varies inversely according to heights of the toner stacks canalso be used to establish level toner stack heights. The respectivecolor toners are deposited one upon the other at respective locations onthe receiver and the height of a respective color toner stack is the sumof the toner heights of each respective color. Uniform stack heightprovides the print with a more even or uniform gloss.

FIGS. 1-2 are elevational cross-sections showing portions of a typicalelectrophotographic printer 100 useful with various embodiments. Printer100 is adapted to produce images, such as single-color images (i.e.,monochrome images), or multicolor images such as CMYK, or pentachrome(five-color) images, on a receiver. Multicolor images are also known as“multi-component” images. One embodiment involves printing using anelectrophotographic print engine having five sets of single-colorimage-producing or image-printing stations or modules arranged intandem, but more or less than five colors can be combined on a singlereceiver. Other electrophotographic writers or printer apparatus canalso be included. Various components of printer 100 are shown asrollers; other configurations are also possible, including belts.

Referring to FIG. 1, printer 100 is an electrophotographic printingapparatus having a number of tandemly-arranged electrophotographicimage-forming printing subsystems 31, 32, 33, 34, 35, also known aselectrophotographic imaging subsystems. Each printing subsystem 31, 32,33, 34, 35 produces a single-color toner image for transfer using arespective transfer subsystem 50 (for clarity, only one is labeled) to areceiver 42 successively moved through the modules. In some embodimentsone or more of the printing subsystem 31, 32, 33, 34, 35 can print acolorless toner image, which can be used to provide a protectiveovercoat or tactile image features. Receiver 42 is transported fromsupply unit 40, which can include active feeding subsystems as known inthe art, into printer 100 using a transport web 81. In variousembodiments, the visible image can be transferred directly from animaging roller to a receiver, or from an imaging roller to one or moretransfer roller(s) or belt(s) in sequence in transfer subsystem 50, andthen to receiver 42. Receiver 42 is, for example, a selected section ofa web or a cut sheet of a planar receiver media such as paper ortransparency film.

In the illustrated embodiments, each receiver 42 can have up to fivesingle-color toner images transferred in registration thereon during asingle pass through the five printing subsystems 31, 32, 33, 34, 35 toform a pentachrome image. As used herein, the term “pentachrome” impliesthat in a print image, combinations of various of the five colors arecombined to form other colors on the receiver at various locations onthe receiver, and that all five colors participate to form processcolors in at least some of the subsets. That is, each of the five colorsof toner can be combined with toner of one or more of the other colorsat a particular location on the receiver to form a color different thanthe colors of the toners combined at that location. In an exemplaryembodiment, printing subsystem 31 forms black (K) print images, printingsubsystem 32 forms yellow (Y) print images, printing subsystem 33 formsmagenta (M) print images, and printing subsystem 34 forms cyan (C) printimages.

Printing subsystem 35 can form a red, blue, green, or other fifth printimage, including an image formed from a clear toner (e.g., one lackingpigment). The four subtractive primary colors, cyan, magenta, yellow,and black, can be combined in various combinations of subsets thereof toform a representative spectrum of colors. The color gamut of a printer(i.e., the range of colors that can be produced by the printer) isdependent upon the materials used and the process used for forming thecolors. The fifth color can therefore be added to improve the colorgamut. In addition to adding to the color gamut, the fifth color canalso be a specialty color toner or spot color, such as for makingproprietary logos or colors that cannot be produced with only CMYKcolors (e.g., metallic, fluorescent, or pearlescent colors), or a cleartoner or tinted toner. Tinted toners absorb less light than theytransmit, but do contain pigments or dyes that move the hue of lightpassing through them towards the hue of the tint. For example, ablue-tinted toner coated on white paper will cause the white paper toappear light blue when viewed under white light, and will cause yellowsprinted under the blue-tinted toner to appear slightly greenish underwhite light.

Receiver 42 a is shown after passing through printing subsystem 31.Print image 38 on receiver 42 a includes unfused toner particles.Subsequent to transfer of the respective print images, overlaid inregistration, one from each of the respective printing subsystems 31,32, 33, 34, 35, receiver 42 a is advanced to a fuser module 60 (i.e., afusing or fixing assembly) to fuse the print image 38 to the receiver 42a. Transport web 81 transports the print-image-carrying receivers to thefuser module 60, which fixes the toner particles to the respectivereceivers, generally by the application of heat and pressure. Thereceivers are serially de-tacked from the transport web 81 to permitthem to feed cleanly into the fuser module 60. The transport web 81 isthen reconditioned for reuse at cleaning station 86 by cleaning andneutralizing the charges on the opposed surfaces of the transport web81. A mechanical cleaning station (not shown) for scraping or vacuumingtoner off transport web 81 can also be used independently or withcleaning station 86. The mechanical cleaning station can be disposedalong the transport web 81 before or after cleaning station 86 in thedirection of rotation of transport web 81.

In the illustrated embodiment, the fuser module 60 includes a heatedfusing roller 62 and an opposing pressure roller 64 that form a fusingnip 66 therebetween. In an embodiment, fuser module 60 also includes arelease fluid application substation 68 that applies release fluid,e.g., silicone oil, to fusing roller 62. Alternatively, wax-containingtoner can be used without applying release fluid to the fusing roller62. Other embodiments of fusers, both contact and non-contact, can beemployed. For example, solvent fixing uses solvents to soften the tonerparticles so they bond with the receiver. Photoflash fusing uses shortbursts of high-frequency electromagnetic radiation (e.g., ultravioletlight) to melt the toner. Radiant fixing uses lower-frequencyelectromagnetic radiation (e.g., infrared light) to more slowly melt thetoner. Microwave fixing uses electromagnetic radiation in the microwaverange to heat the receivers (primarily), thereby causing the tonerparticles to melt by heat conduction, so that the toner is fixed to thereceiver.

The fused receivers (e.g., receiver 42 b carrying fused image 39) aretransported in series from the fuser module 60 along a path either to anoutput tray 69, or back to printing subsystems 31, 32, 33, 34, 35 toform an image on the backside of the receiver (i.e., to form a duplexprint). Receivers 42 b can also be transported to any suitable outputaccessory. For example, an auxiliary fuser or glossing assembly canprovide a clear-toner overcoat. Printer 100 can also include multiplefuser modules 60 to support applications such as overprinting, as knownin the art.

In various embodiments, between the fuser module 60 and the output tray69, receiver 42 b passes through a finisher 70. Finisher 70 performsvarious paper-handling operations, such as folding, stapling,saddle-stitching, collating, and binding.

Printer 100 includes main printer apparatus logic and control unit (LCU)99, which receives input signals from various sensors associated withprinter 100 and sends control signals to various components of printer100. LCU 99 can include a microprocessor incorporating suitable look-uptables and control software executable by the LCU 99. It can alsoinclude a field-programmable gate array (FPGA), programmable logicdevice (PLD), programmable logic controller (PLC) (with a program in,e.g., ladder logic), microcontroller, or other digital control system.LCU 99 can include memory for storing control software and data. In someembodiments, sensors associated with the fuser module 60 provideappropriate signals to the LCU 99. In response to the sensor signals,the LCU 99 issues command and control signals that adjust the heat orpressure within fusing nip 66 and other operating parameters of fusermodule 60. This permits printer 100 to print on receivers of variousthicknesses and surface finishes, such as glossy or matte.

FIG. 2 shows additional details of printing subsystem 31, which isrepresentative of printing subsystems 32, 33, 34, and 35 (FIG. 1).Photoreceptor 206 of imaging member 111 includes a photoconductive layerformed on an electrically conductive substrate. The photoconductivelayer is an insulator in the substantial absence of light so thatelectric charges are retained on its surface. Upon exposure to light,the charge is dissipated. In various embodiments, photoreceptor 206 ispart of, or disposed over, the surface of imaging member 111, which canbe a plate, drum, or belt. Photoreceptors can include a homogeneouslayer of a single material such as vitreous selenium or a compositelayer containing a photoconductor and another material. Photoreceptors206 can also contain multiple layers.

Charging subsystem 210 applies a uniform electrostatic charge tophotoreceptor 206 of imaging member 111. In an exemplary embodiment,charging subsystem 210 includes a wire grid 213 having a selectedvoltage. Additional necessary components provided for control can beassembled about the various process elements of the respective printingsubsystems. Meter 211 measures the uniform electrostatic charge providedby charging subsystem 210.

An exposure subsystem 220 is provided for selectively modulating theuniform electrostatic charge on photoreceptor 206 in an image-wisefashion by exposing photoreceptor 206 to electromagnetic radiation toform a latent electrostatic image. The uniformly-charged photoreceptor206 is typically exposed to actinic radiation provided by selectivelyactivating particular light sources in an LED array or a laser deviceoutputting light directed onto photoreceptor 206. In embodiments usinglaser devices, a rotating polygon (not shown) is sometimes used to scanone or more laser beam(s) across the photoreceptor in the fast-scandirection. One pixel site is exposed at a time, and the intensity orduty cycle of the laser beam is varied at each dot site. In embodimentsusing an LED array, the array can include a plurality of LEDs arrangednext to each other in a line, all dot sites in one row of dot sites onthe photoreceptor can be selectively exposed simultaneously, and theintensity or duty cycle of each LED can be varied within a line exposuretime to expose each pixel site in the row during that line exposuretime.

As used herein, an “engine pixel” is the smallest addressable unit onphotoreceptor 206 which the exposure subsystem 220 (e.g., the laser orthe LED) can expose with a selected exposure different from the exposureof another engine pixel. Engine pixels can overlap (e.g., to increaseaddressability in the slow-scan direction). Each engine pixel has acorresponding engine pixel location, and the exposure applied to theengine pixel location is described by an engine pixel level.

The exposure subsystem 220 can be a write-white or write-black system.In a write-white or “charged-area-development” system, the exposuredissipates charge on areas of photoreceptor 206 to which toner shouldnot adhere. Toner particles are charged to be attracted to the chargeremaining on photoreceptor 206. The exposed areas therefore correspondto white areas of a printed page. In a write-black or “discharged-areadevelopment” system, the toner is charged to be attracted to a biasvoltage applied to photoreceptor 206 and repelled from the charge onphotoreceptor 206. Therefore, toner adheres to areas where the charge onphotoreceptor 206 has been dissipated by exposure. The exposed areastherefore correspond to black areas of a printed page.

In the illustrated embodiment, meter 212 is provided to measure thepost-exposure surface potential within a patch area of a latent imageformed from time to time in a non-image area on photoreceptor 206. Othermeters and components can also be included (not shown).

A development station 225 includes toning shell 226, which can berotating or stationary, for applying toner of a selected color to thelatent image on photoreceptor 206 to produce a developed image onphotoreceptor 206 corresponding to the color of toner deposited at thisprinting subsystem 31. Development station 225 is electrically biased bya suitable respective voltage to develop the respective latent image,which voltage can be supplied by a power supply (not shown). Developeris provided to toning shell 226 by a supply system (not shown) such as asupply roller, auger, or belt. Toner is transferred by electrostaticforces from development station 225 to photoreceptor 206. These forcescan include Coulombic forces between charged toner particles and thecharged electrostatic latent image, and Lorentz forces on the chargedtoner particles due to the electric field produced by the bias voltages.

In some embodiments, the development station 225 employs a two-componentdeveloper that includes toner particles and magnetic carrier particles.The exemplary development station 225 includes a magnetic core 227 tocause the magnetic carrier particles near toning shell 226 to form a“magnetic brush,” as known in the electrophotographic art. Magnetic core227 can be stationary or rotating, and can rotate with a speed anddirection the same as or different than the speed and direction oftoning shell 226. Magnetic core 227 can be cylindrical ornon-cylindrical, and can include a single magnet or a plurality ofmagnets or magnetic poles disposed around the circumference of magneticcore 227. Alternatively, magnetic core 227 can include an array ofsolenoids driven to provide a magnetic field of alternating direction.Magnetic core 227 preferably provides a magnetic field of varyingmagnitude and direction around the outer circumference of toning shell226. Development station 225 can also employ a mono-component developercomprising toner, either magnetic or non-magnetic, without separatemagnetic carrier particles.

Transfer subsystem 50 includes transfer backup member 113, andintermediate transfer member 112 for transferring the respective printimage from photoreceptor 206 of imaging member 111 through a firsttransfer nip 201 to surface 216 of intermediate transfer member 112, andthence to a receiver 42 which receives respective toned print images 38from each printing subsystem in superposition to form a composite imagethereon. The print image 38 is, for example, a separation of one color,such as cyan. Receiver 42 is transported by transport web 81. Transferto a receiver is effected by an electrical field provided to transferbackup member 113 by power source 240, which is controlled by LCU 99.Receiver 42 can be any object or surface onto which toner can betransferred from imaging member 111 by application of the electricfield. In this example, receiver 42 is shown prior to entry into asecond transfer nip 202, and receiver 42 a is shown subsequent totransfer of the print image 38 onto receiver 42 a.

In the illustrated embodiment, the toner image is transferred from thephotoreceptor 206 to the intermediate transfer member 112, and fromthere to the receiver 42. Registration of the separate toner images isachieved by registering the separate toner images on the receiver 42, asis done with the NexPress 2100. In some embodiments, a single transfermember is used to sequentially transfer toner images from each colorchannel to the receiver 42. In other embodiments, the separate tonerimages can be transferred in register directly from the photoreceptor206 in the respective printing subsystem 31, 32, 33, 34, 25 to thereceiver 42 without using a transfer member. Either transfer process issuitable when practicing this invention. An alternative method oftransferring toner images involves transferring the separate tonerimages, in register, to a transfer member and then transferring theregistered image to a receiver.

LCU 99 sends control signals to the charging subsystem 210, the exposuresubsystem 220, and the respective development station 225 of eachprinting subsystem 31, 32, 33, 34, 35 (FIG. 1), among other components.Each printing subsystem can also have its own respective controller (notshown) coupled to LCU 99.

Various finishing systems can be used to apply features such asprotection, glossing, or binding to the printed images. The finishingsystem scan be implemented as an integral components of the printer 100,or can include one or more separate machines through which the printedimages are fed after they are printed.

FIG. 3 shows a conventional processing path that can be used to producea printed image 450 using a print engine 370. A pre-processing system305 is used to process a page description file 300 to provide image data350 that is in a form that is ready to be printed by the print engine370. In an exemplary configuration, the pre-processing system 305includes a digital front end (DFE) 310 and an image processing module330. The pre-processing system 305 can be a part of printer 100 (FIG.1), or may be a separate system which is remote from the printer 100.The DFE 310 and an image processing module 330 can each include one ormore suitably-programmed computer or logic devices adapted to performoperations appropriate to provide the image data 350.

The DFE 310 receives page description files 300 which define the pagesthat are to be printed. The page description files 300 can be in anyappropriate format (e.g., the well-known Postscript command file formator the PDF file format) that specifies the content of a page in terms oftext, graphics and image objects. The image objects are typicallyprovided by input devices such as scanners, digital cameras or computergenerated graphics systems. The page description file 300 can alsospecify invisible content such as specifications of texture, gloss orprotective coating patterns.

The DFE 310 rasterizes the page description file 300 into image bitmapsfor the print engine to print. The DFE 310 can include variousprocessors, such as a raster image processor (RIP) 315, a colortransform processor 320 and a compression processor 325. It can alsoinclude other processors not shown in FIG. 3, such as an imagepositioning processor or an image storage processor. In someembodiments, the DFE 310 enables a human operator to set up parameterssuch as layout, font, color, media type or post-finishing options.

The RIP 315 rasterizes the objects in the page description file 300 intoan image bitmap including an array of image pixels at an imageresolution that is appropriate for the print engine 370. For text orgraphics objects the RIP 315 will create the image bitmap based on theobject definitions. For image objects, the RIP 315 will resample theimage data to the desired image resolution.

The color transform processor 320 will transform the image data to thecolor space required by the print engine 370, providing colorseparations for each of the color channels (e.g., CMYK). For cases wherethe print engine 370 includes one or more additional colors (e.g., red,blue, green, gray or clear), the color transform processor 320 will alsoprovide color separations for each of the additional color channels. Theobjects defined in the page description file 300 can be in anyappropriate input color space such as sRGB, CIELAB, PCS LAB or CMYK. Insome cases, different objects may be defined using different colorspaces. The color transform processor 320 applies an appropriate colortransform to convert the objects to the device-dependent color space ofthe print engine 370. Methods for creating such color transforms arewell-known in the color management art, and any such method can be usedin accordance with the present invention. Typically, the colortransforms are defined using color management profiles that includemulti-dimensional look-up tables. Input color profiles are used todefine a relationship between the input color space and a profileconnection space (PCS) defined for a color management system (e.g., thewell-known ICC PCS associated with the ICC color management system).Output color profiles define a relationship between the PCS and thedevice-dependent output color space for the printer 100. The colortransform processor 320 transforms the image data using the colormanagement profiles. Typically, the output of the color transformprocessor 320 will be a set of color separations including an array ofpixels for each of the color channels of the print engine 370 stored inmemory buffers.

The processing applied in digital front end 310 can also include otheroperations not shown in FIG. 3. For example, in some configurations, theDFE 310 can apply the halo correction process described incommonly-assigned U.S. Pat. No. 9,147,232 (Kuo) entitled “Reducing haloartifacts in electrophotographic printing systems,” which isincorporated herein by reference.

The image data provided by the digital front end 310 is sent to theimage processing module 330 for further processing. In order to reducethe time needed to transmit the image data, a compressor processor 325is typically used to compress the image data using an appropriatecompression algorithm. In some cases, different compression algorithmscan be applied to different portions of the image data. For example, alossy compression algorithm (e.g., the well-known JPEG algorithm) can beapplied to portions of the image data including image objects, and alossless compression algorithm can be applied to portions of the imagedata including binary text and graphics objects. The compressed imagevalues are then transmitted over a data link to the image processingmodule 330, where they are decompressed using a decompression processor335 which applies corresponding decompression algorithms to thecompressed image data.

A halftone processor 340 is used to apply a halftoning process to theimage data. The halftone processor 340 can apply any appropriatehalftoning process known in the art. Within the context of the presentdisclosure, halftoning processes are applied to a continuous-tone imageto provide an image having a halftone dot structure appropriate forprinting using the printer module 435. The output of the halftoning canbe a binary image or a multi-level image. In an exemplary configuration,the halftone processor 340 applies the halftoning process described incommonly assigned U.S. Pat. No. 7,830,569 (Tai et al.), entitled“Multilevel halftone screen and sets thereof,” which is incorporatedherein by reference. For this halftoning process, a three-dimensionalhalftone screen is provided that includes a plurality of planes, eachcorresponding to one or more intensity levels of the input image data.Each plane defines a pattern of output exposure intensity valuescorresponding to the desired halftone pattern. The halftoned pixelvalues are multi-level values at the bit depth appropriate for the printengine 370.

The image enhancement processor 345 can apply a variety of imageprocessing operations. For example, an image enhancement processor 345can be used to apply various image enhancement operations. In someconfigurations, the image enhancement processor 345 can apply analgorithm that modifies the halftone process in edge regions of theimage (see U.S. Pat. No. 7,079,281, entitled “Edge enhancement processorand method with adjustable threshold setting” and U.S. Pat. No.7,079,287 entitled “Edge enhancement of gray level images” (both to Nget al.), and both of which are incorporated herein by reference).

The pre-processing system 305 provides the image data 350 to the printengine 370, where it is printed to provide the printed image 450. Thepre-processing system 305 can also provide various signals to the printengine 370 to control the timing at which the image data 350 is printedby the print engine 370. For example, the pre-processing system 305 cansignal the print engine 370 to start printing when a sufficient numberof lines of image data 350 have been processed and buffered to ensurethat the pre-processing system 305 will be capable of keeping up withthe rate at which the print engine 370 can print the image data 350.

A data interface 405 in the print engine 370 receives the data from thepre-processing system 305. The data interface 405 can use any type ofcommunication protocol known in the art, such as standard Ethernetnetwork connections. A printer module controller 430 controls theprinter module 435 in accordance with the received image data 350. In anexemplary configuration, the printer module 435 can be the printer 100of FIG. 1, which includes a plurality of individual electrophotographicprinting subsystems 31, 32, 33, 34, 35 for each of the color channels.For example, the printer module controller 430 can provide appropriatecontrol signals to activate light sources in the exposure subsystem 220(FIG. 2) to exposure the photoreceptor 206 with an exposure pattern. Insome configurations, the printer module controller 430 can apply variousimage enhancement operations to the image data. For example, analgorithm can be applied to compensate for various sources ofnon-uniformity in the printer 100 (e.g., streaks formed in the chargingsubsystem 210, the exposure subsystem 220, the development station 225or the fuser module 60. One such compensation algorithm is described incommonly-assigned U.S. Pat. No. 8,824,907 (Kuo et al.), entitled“Electrophotographic printing with column-dependent tonescaleadjustment,” which is incorporated herein by reference.

In the configuration of FIG. 3, the pre-processing system 305 is tightlycoupled to the print engine 370 in that it must supply image data 350 ina state which is matched to the printer resolution and the halftoningstate required for the printer module 435. As a result, when newversions of the print engine 370 having different printer resolutions orhalftone state requirements are developed, it has been necessary to alsoprovide an updated version of the pre-processing system 305 thatprovides image data 350 in an appropriate state. This has thedisadvantage that customers are required to upgrade both thepre-processing system 305 and the print engine 370 at the same time,both of which can have significant costs. The present inventionaddresses this problem by providing an improved print engine designwhich is compatible with a variety of different pre-processing systems.

FIG. 4 shows an improved print engine 400 as described incommonly-assigned U.S. Pat. No. 10,062,017 to C. H. Kuo et al., entitled“Print engine with adaptive processing,” which is incorporated herein byreference. The improved print engine 400 is adapted to produce printedimages 450 from image data 350 provided by a plurality of differentpre-processing systems 305 that are configured to supply image data 350having different image resolutions and halftoning states. In anexemplary configuration, the pre-processing systems 305 are similar tothat discussed with respect to FIG. 3, and includes a digital front end310 and an image processing module 330. Details of the processingprovided by the digital front end 310 and an image processing module 330are not included in FIG. 4 for clarity, but will be analogous to theprocessing operations that were discussed with respect to FIG. 3. Inthis case, in addition to supplying image data 350, the pre-processingsystem 305 also supplies appropriate metadata 360 that provides anindication of the state of the image data 350. In particular, themetadata 360 provides an indication of the image resolution and thehalftoning state of the image data 350.

In an exemplary configuration, the metadata 360 includes an imageresolution parameter that provides an indication of an image resolutionof the image data 350 provided by the pre-processing system 305 and ahalftone state parameter that provides an indication of a halftoningstate of the image data provided by the pre-processing system 305.

The image resolution parameter (R) can take any appropriate form thatconveys information about the image resolution of the image data 350. Insome embodiments, the image resolution parameter can be an integerspecifying the spatial resolution in appropriate units such as dots/inch(dpi) (e.g., R=600 for 600 dpi and R=1200 for 1200 dpi). In otherembodiments, the image resolution parameter can be an index to anenumerated list of allowable spatial resolutions (e.g., R=0 for 600 dpiand R=1 for 1200 dpi).

The halftone state parameter (H) can also take any appropriate form. Insome embodiments, the halftone state parameter can be a Boolean variableindicating whether or not a halftoning process was applied in thepre-processing system 305 such that the image data 350 is in a halftonedstate (e.g., H=FALSE indicates that a halftoning process was not appliedso that the image data 350 is in a continuous tone state, and H=TRUEindicates that a halftoning process was applied sot that the image data350 is in a halftoned state.) In other embodiments, when thepre-processing system 305 applied a halftoning process, the halftonestate parameter can also convey additional information about the type ofhalftoning process that was applied. For example, the halftone stateparameter can be an integer variable, where H=0 indicates that nohalftoning process was applied, and other integer values represent anindex to an enumerated list of available halftoning states (e.g.,different screen frequency/angle/dot shape combinations).

The metadata 360 can also specify other relevant pieces of information.For example, for the case where the image data 350 is in a continuoustone state such that a halftone processor 425 in the print engine 400will be required to apply a halftoning operation, the metadata 360 canalso include one or more halftoning parameters that are used by thehalftone processor 425 to control the halftoning operation. In someembodiments, the halftoning parameters can include a screen angleparameter, a screen frequency parameter, or a screen type parameter. Inother embodiments, the halftoning parameters can include a halftoneconfiguration index that is used to select one of a predefined set ofhalftone algorithm configurations.

The print engine 400 receives the image data 350 and the metadata 360using an appropriate data interface 405 (e.g., an Ethernet interface).The print engine includes a metadata interpreter 410 that analyzes themetadata 360 to provide appropriate control signals 415 that are used tocontrol a resolution modification processor 420 and a halftone processor425, which are used to process the image data 350 to provide processedimage data 428, which is in an appropriate state to be printed by theprinter module 435. Printer module controller 430 then controls theprinter module 435 to print the processed image data 428 to produce theprinted image 450 in an analogous manner to that which was discussedrelative to FIG. 3.

FIG. 5 shows additional details of the resolution modification processor420 and the halftone processor 425 of FIG. 4 according to an exemplaryconfiguration. In this example, the control signals 415 provided by themetadata interpreter 410 (FIG. 4) in response to analyzing the metadata360 (FIG. 4) include a resolution modification flag 416, a resize factor417, a halftoning flag 418 and halftoning parameters 419.

The resolution modification flag 416 provides an indication of whether aresolution modification must be performed. In an exemplary configurationthe resolution modification flag 416 is a Boolean variable that would beset to FALSE if no resolution modification is required (i.e., if theimage resolution of the image data 350 matches the printer resolution ofthe printer module 435), and would be set to TRUE of a resolutionmodification is required.

The halftoning flag 418 provides an indication of whether a halftoningoperation is required. In an exemplary configuration the halftone flag418 is a Boolean variable that would be set to FALSE if no halftoningoperation is required (i.e., if the image data 350 is in a halftoningstate that is appropriate for the printer module 435), and would be setto TRUE if a halftoning operation must be applied to the image data 350before it is ready to be printed.

The resolution modification processor 420 applies modify resolution test421 to determine whether a resolution modification should be performedresponsive to the resolution modification flag 416. If a resolutionmodification is required, a resolution modification operation 422 isperformed. In some configurations, the metadata interpreter 410 (FIG. 4)provides a resize factor 417 that specifies the amount of resizing thatmust be provided to adjust the resolution of the image data 350 to theresolution required by the printer module 435 (FIG. 4). In someconfigurations, the resize factor 417 is a variable specifying the ratiobetween the printer resolution and the image resolution. For example, ifthe image data 350 is at 600 dpi and the printer module 435 prints at1200 dpi, the resize factor 417 would specify that a 2× resolutionmodification is required. In various configurations the resize factor417 could be greater than 1.0 if the printer module 435 has a higherresolution than the image data 350, or it could be less than 1.0 if theprinter module 435 has a lower resolution than the image data 350.

In an exemplary configuration, if the image resolution of the image data350 supplied by the pre-processing system 305 is an integer fraction ofthe printer resolution of the printer module 435 so that the resizefactor 417 is a positive integer, the resolution modification operation422 performs the resolution modification by performing a pixelreplication process. For example, each 600 dpi image pixel in the imagedata 350 would be replaced with a 2×2 array of 1200 dpi image pixels,each having the same pixel value. In other configurations, anappropriate interpolation process can be used by the resolutionmodification operation 422 (e.g., nearest neighbor interpolation,bi-linear interpolation or bi-cubic interpolation). The use of aninterpolation algorithm is particularly useful of the resize factor isnot an integer.

For cases where the resize factor is less than 1.0, the resolutionmodification operation 422 can perform appropriate averaging operationsto avoid aliasing artifacts. For example, if the resize factor 417 is0.5, then 2×2 blocks of image pixels in the image data 350 can beaveraged together to provide the new resolution. In otherconfigurations, the resolution modification operation 422 can apply alow-pass filter operation followed by a resampling operation.

The halftone processor 425 applies halftone image test 426 to determinewhether a halftoning operation should be performed responsive to thehalftoning flag 418. If a halftoning operation is required (e.g., if theimage data 350 is in a continuous-tone state), a halftoning operation427 is performed. In some configurations, the metadata interpreter 410(FIG. 4) provides one or more halftoning parameters 419 that are used tocontrol the halftoning operation. As discussed earlier, the halftoningparameters 419 can include a screen angle parameter, a screen frequencyparameter, or a screen type parameter. In other embodiments, thehalftoning parameters 419 can include a halftone configuration indexthat is used to select one of a predefined set of halftone algorithmconfigurations

The halftoning operation applied by the halftone processor 425 can useany appropriate halftoning algorithm known in the art. In someembodiments, any of the halftoning algorithms described incommonly-assigned U.S. Pat. No. 7,218,420 (Tai et al.), entitled “Graylevel halftone processing,” commonly-assigned U.S. Pat. No. 7,626,730(Tai et al.), entitled “Method of making a multilevel halftone screen,”and commonly-assigned U.S. Pat. No. 7,830,569 (Tai et al.), entitled,“Multilevel halftone screen and sets thereof,” each of which areincorporated herein by reference, can be used. Such halftoningalgorithms typically involve defining look-up tables defining thehalftone dot shape as a function of position for a tile of pixels.Different look-up tables can be specified to produce different halftonedot patterns. For example, different look-up tables can be specified fordifferent screen angles, screen frequencies and dot shapes. In thiscase, the halftoning parameters 419 can include a halftone configurationindex that selects which look-up table should be used to halftone theimage data 350. In a preferred configuration, the halftone processor 425uses a computational halftone process to compute halftoned pixel valuesusing a defined set of calculations. An exemplary computational halftoneprocess that can be used in accordance with the present invention isdescribed in the aforementioned U.S. Pat. No. 10,062,017.

Consider the case where the printer module 435 prints halftoned imagedata at 1200 dpi, but where different pre-processing systems 305 andconfigurations can be used to supply image data 350 at either 600 dpi or1200 dpi, and in either a halftoned state or a continuous tone state. Inthis case, there will be four different combinations of the imageresolution parameters and the halftone state parameters that the printengine must deal with.

-   1. The image resolution parameter indicates that image data 350 is    600 dpi, and the halftone state parameter indicates that the image    data 350 is in a halftoned state. In this case, the print engine 400    would print the image data 350 in a mode that emulates a 600 dpi    printer. The resolution modification processor 420 would be used to    modify the image resolution to provide the 1200 dpi data required by    the printer module 435. In an exemplary embodiment, each 600 dpi    image pixel is replicated to provide a 2×2 array of 1200 dpi image    pixels. Since the image data is already in a halftoned state, the    halftone operation 427 would be bypassed.-   2. The image resolution parameter indicates that the image data 350    is 600 dpi, and the halftone state parameter indicates that the    image data 350 is in a continuous-tone state. In this case, the    resolution modification processor 420 would be used to modify the    image resolution to provide the 1200 dpi data appropriate for the    printer module 435, and the halftone processor 425 would apply a    halftoning operation 427 to the 1200 dpi image data in accordance    with the halftoning parameters 419.-   3. The image resolution parameter indicates that the image data 350    is 1200 dpi, and the halftone state parameter indicates that the    image data 350 is in a halftoned state. In this case, the image data    350 is already in a state that is ready for printing by the printer    module 435, therefore the resolution modification operation 422 and    the halftoning operation 427 would both be bypassed.-   4. The image resolution parameter indicates that image data 350 is    1200 dpi, and the halftone state parameter indicates that the image    data 350 is in a continuous-tone state. In this case, since the    image data is already at 1200 dpi so that the resolution    modification operation 422 would be bypassed, and the halftone    processor 425 would apply a halftoning operation 427 to the 1200 dpi    image data in accordance with the halftoning parameters 419.

The exposure subsystem 220 (FIG. 2) in each printing subsystem 31, 32,33, 34, 35 (FIG. 1) typically includes a printhead 475 having a lineararray of light sources 460 as illustrated in FIG. 6A. In an exemplaryembodiment, the light sources 460 are LED light sources, although othertypes of light sources such as laser diodes can also be used. In theillustrated configuration, the printhead 475 is fabricated using threelight source tiles 470, each of which includes fifteen light sourcechips 465. The light source chips 465 include a linear array of 384individual light sources 460 as illustrated in FIG. 6B. Each of thelight sources 460 is connected to a corresponding connection pad 466through which an electrical signal is provided to selectively activatethe light source 460 in accordance with image data. The light sources460 have a width W_(S), a height H_(S) and a light source pitch (i.e., alight source-to-light source spacing) P_(S). In an exemplaryconfiguration, W_(S)=12 μm, H_(S)=15 μm and P_(S)=21.15 μm(corresponding to 1200 dots/inch).

The light source chips 465 are positioned end-to-end in the printhead475 to form a single array of 384×15×3=17,280 light sources 460.Ideally, each of the light sources 460 are spaced with an identicalspacing P_(S), such that they expose the photoreceptor 206 in apredictable location. However, in practice there will be a number ofsources of variability that can introduce cross-track position errors inthe exposed pixels relative to their expected positions. Sources ofcross-track position errors can include variations in the light sourcepitch P_(S) within a light source chip 465, variation in the length ofthe light source chips 465, placement errors in the position of thelight source chips 465 within a light source tile 470, variation in thelength of the light source tiles 470, placement errors in the positionof the light source tiles 470 within the printhead 475, and placementerrors in the position of the printhead 475. Additionally, the lightsources 460 in the printhead 475 are typically imaged onto thephotoreceptor 206 with an array of micro-lenses. The micro-lenses aretypically gradient index “SELFOC” lens rods. Variations in the positionand orientation of the micro-lenses can also introduce variability inthe position of the image of the light sources 460 on the photoreceptor206, which will combine with the other sources of variation.

Cross-track position errors for the light sources 460 in the printhead475 can be particularly problematic when they differ from one printingsubsystem 31, 32, 33, 34, 35 to another, resulting in color-to-coloralignment errors which can be visible and objectionable in manyinstances. To provide acceptable alignment, the color-to-color alignmenterrors should typically be less than 40 μm, and more preferably shouldbe less than 20 μm. However, with typical manufacturing tolerances,alignment errors as large as 200 μm have been observed. Therefore, thereis a need for a method to characterize and correct for the cross-trackposition errors that can be implemented without the need for complex andcostly fixtures.

As described in commonly-assigned, co-pending U.S. patent applicationSer. No. 16/417,731, entitled: “Correcting cross-track errors in alinear printhead”, by Kuo et al., which is incorporated herein byreference, FIG. 7 shows a flowchart of a method for determining aposition correction function 555 that characterizes the cross-trackposition errors associated with a printhead 475 (FIG. 6A) in accordancewith an exemplary embodiment. The method includes providing digitalimage data for a test target 500. The test target 500 preferablyincludes a plurality of alignment marks 570 positioned at predefinedcross-track positions as illustrated in the exemplary arrangement shownin FIG. 8. The alignment marks 570 are preferably distributed along thelength of the printhead 475 which spans the test target 500 in across-track direction 590. The test target 500 may optionally includeother content such as solid patches 575 that can be used for othercalibration or characterization purposes. In an exemplary arrangement,the test target 500 includes alignment marks 570 for a plurality ofdifferent color channels. In the illustrated example, the test targetincludes first color channel image content 580 for a first color channelprinted by a first printing subsystem 31 (FIG. 1), second color channelimage content 581 for a second color channel printed by a secondprinting subsystem 32 (FIG. 1), third color channel image content 582for a third color channel printed by a third printing subsystem 32 (FIG.1), and fourth color channel image content 583 for a fourth colorchannel printed by a fourth printing subsystem 34 (FIG. 1). The imagecontent for each color channel is provided in different image regionsdistributed in the in-track direction 595. The different color channelscan be, for example, black, cyan, magenta and yellow. However, oneskilled in the art will recognize that the color channels can use othercolorants as well. Each of different image regions includes acorresponding set of alignment marks 570. In other embodiments, ratherthan using a single test target 500 including alignment marks 570 forall of the color channels, they can be included in a plurality of testtargets 500 (e.g., one for each color channel).

In the illustrated example of FIG. 8, the alignment marks 570 arepictured as an array of equally spaced vertical lines. However, oneskilled in the art will recognize that there are a wide variety ofdifferent alignment mark spacings and geometries that could be used inaccordance with the present invention. In some configurations, the widthor the cross-track position of the vertical lines can be varied alongthe length of the line in order to enable the centroid of the printedline to be more accurately measured. In other cases, the alignment markscould include crossed lines, circles, diamonds, squares or any othergeometric shape that can be analyzed to determine a cross-track positionof the alignment marks.

In an exemplary arrangement, an alignment marks 570 are provided inproximity to the boundaries between adjacent light source chips 465 inthe printhead 475 (FIG. 6A). This reflects the fact that the most commonsources of position errors relate to length variability and positioningerrors for the light source chips 465 and light source tiles 470.Therefore, forty-four alignment marks 570 would be used for a printhead475 that includes three light source tiles, each including fifteen lightsource chips 465. Preferably, at least ten alignment marks 570 areprovided across the length of the printhead 475 to enable thecharacterization and correction of localized, non-linear cross-trackalignment errors.

Returning to a discussion of FIG. 7, a print test target step 505 isused to print the test target 500 to produce a printed test target 510.In a preferred embodiment, the printed test target 510 is formed on apiece of receiver 42 (FIG. 2) such as a sheet of paper. In other cases,the printed test target 510 can be an image transferred directly ontothe transport web 81 rather than onto a sheet of receiver 42. In otherembodiments, the printed test target 510 can correspond to anintermediate image formed on the surface of the imaging member 111(i.e., the photoreceptor 206) or the surface 216 of an intermediatetransfer member 112 (see FIG. 2).

A capture image step 515 is next used to capture a digital image of theprinted test target 510 using a digital image capture system to providea captured image 520. In an exemplary embodiment, the digital imagecapture system is a flatbed scanner external to the printer 100 which isused to scan the printed test target 510 formed on a receiver 42 afterit has been completely printed and fused. In other embodiments, adigital image capture system (e.g., a digital scanner system or adigital camera system) which is integrated into the printer 100 can beused to capture an image of the printed test target 510 on the receiver42 while the receiver 42 is traveling through the printer 100 (e.g.,while it is being carried on the transport web 81), or before it hasbeen transferred to the receiver 42 (e.g., on surface of the imagingmember 111 or the intermediate transfer member 112).

Next, an analyze captured image step 525 is used to automaticallyanalyze the captured image 520 to determine measured alignment markpositions 530. The measured alignment mark positions 530 include atleast the cross-track positions of the alignment marks 570 in the testtarget 500. In some embodiments the measured alignment mark positions530 can also include the in-track positions of the alignment marks 570.(The in-track positions of the alignment marks 570 can be utilized tocorrect for artifacts such as substrate skew.) In an exemplaryembodiment, a plurality of image lines in the captured image 520 areidentified which intersect the alignment marks 570. The image lines areaveraged to determine a combined image trace which includes tracesthrough the individual alignment marks 570. Equivalently, a low-passfilter can be applied to the image data to average the pixel values in arange of in-track positions, and the combined image trace can bedetermined by taking a single trace through the filtered image.Preferably, any skew in the captured image 520 can be characterized(e.g., by detecting the boundaries of the solid patches 575) andaccounted for in the image analysis process. For example, the capturedimage 520 can be rotated to remove the skew. Alternatively, the imagetraces can be taken along lines parallel to the skew angle, or the imagecan be filtered using a low-pass filter which is rotated by the skewangle.

The combined image trace can then be analyzed to determine the measuredalignment mark positions 530. FIG. 9A shows an example of a combinedimage trace 526, which includes alignment mark profiles 527 for each ofthe alignment marks 570. The “scanner code values” on the y-axis havebeen inverted such that “0” is white and “255” is black. The measuredalignment mark positions 530 for each of the alignment marks can then bedetermined by computing a quantity corresponding to a measure of thecentral tendency for each of the alignment mark profiles 527. Forexample, the measure of the central tendency can be the centroid (i.e.,the mean), the median or the mode of the alignment mark profile 527.

In an exemplary embodiment, an idealized profile function 528 is fit tothe alignment mark profile 527 as illustrated in FIG. 9B. The alignmentmark profile 527 in this figure corresponds to the circled alignmentmark profile 527 in FIG. 9A, and has been shifted to remove the densityof the paper. A Gaussian function was then fit to the alignment markprofile 527 to determine the idealized profile function 528. Themeasured alignment mark position 530 is then determined by computing themeasure of central tendency (i.e., the centroid) of the idealizedprofile function 528. This approach has the advantage that it is lesssusceptible to noise in the image data.

Next, a determine cross-track position errors step 540 is used todetermine cross-track position errors 545 by comparing the measuredalignment mark positions 530 with corresponding reference alignment markpositions 535. In some embodiments, the reference alignment markpositions 535 can correspond to ideal positions of the alignment marks570 determined from their positions in the original test target 500. Ina preferred embodiment, one of the color channels is designated to be areference color channel, and the other color channels are designated tobe non-reference color channels. In this case, the measured alignmentmark positions 530 for the reference color channel are used as thereference alignment mark positions 535 for the non-reference colorchannels. In this way, the cross-track position errors 545 for thenon-reference color channels correspond to cross-track differencesbetween the image content printed in the non-reference color channel andthe reference color channel. In some configurations, a predefined colorchannel (e.g., the black color channel) is designated to be thereference color channel. In other cases, it can be advantageous todesignate the color channel that has the largest cross-track line length(e.g., the color channel having the largest cross-track distance betweenthe first and last alignment marks) to be the reference color channel.In this case, the position corrections that are applied to thenon-reference color channels will stretch out the image data (e.g., byrepeating certain image pixels) rather than shortening the image data(e.g., by deleting certain image pixels). This eliminates thepossibility that a portion of a single-pixel wide line might be erasedby deleting the corresponding image pixels.

FIG. 10A illustrates the cross-track position errors 545 determined fora printed test target 510 produced using an exemplary printhead 475. Thecross-track position errors 545 were determined by computing thedifference between the measured alignment mark positions 530 and thecorresponding reference alignment mark positions 535. A positivecross-track position error 545 corresponds to the case where theposition of the alignment mark in the printed image is longer than thereference position (i.e., to the right), and a negative cross-trackposition error 545 corresponds to the case where the printed image isshorter than the reference position (i.e., to the left). It can be seenin this example, that a portion of the printhead has negativecross-track position errors, while another portion of the printhead haspositive cross-track position errors, indicating that the spacingbetween the light sources varies across the width of the printhead.

A determine position correction function step 550 is then used todetermine a position correction function 555 based on the measuredcross-track position errors 545. The position errors in this example arescaled by the output pixel spacing so that they represented in terms ofthe number of output pixels (e.g., the number of 1200 dpi pixels). In anexemplary embodiment, a smooth function is fit to the measuredcross-track position errors 545 to determine a cross-track positionerror function 546. For example, the cross-track position error function546 can be determined by fitting a smoothing spline or a polynomialfunction to the measured cross-track position errors 545. Such smoothingoperations are well-known to those skilled in the art.

In an exemplary embodiment, corrections are applied by resampling theimage data. In this case, the resampling operation effectively shiftsthe image data as a function of pixel position by an integer number ofoutput pixels. The required shift can be determined by quantizing thecross-track position error function 546 to determine a quantizedcross-track position error function 547. The quantized cross-trackposition error function 547 gives an indication of how many pixels tothe right or left the output pixel position has been shifted. Forexample, the quantized position error for pixel indices in the range of1357-6441 are one pixel to the left of their expected positions.

In order to correct for the cross-track position errors, a positioncorrection function 555 can be determined by inverting the quantizedcross-track position error function 547 as shown in FIG. 10B. In anexemplary embodiment, the correction is applied by resampling the imagedata at shifted pixel positions. The position correction function 555gives an indication of how many output pixels the image data should beshifted as a function of cross-track pixel position.

A representation of the position correction function 555 can be storedin a digital memory in any appropriate format to be used in thecorrection of digital image data. For example, the full positioncorrection function 555 can be stored in the digital memory, either in aquantized form such as that illustrated in FIG. 10B, or in anunquantized form. Alternatively, the position correction function 555can be represented in other formats. For example, the quantized positioncorrection function 555 of FIG. 10B can be fully represented by storingthe differences between the quantized position correction values atsequential pixel positions. An example of such a position correctionfunction representation 560 is illustrated in FIG. 10C. The positioncorrection function representation 560 can be stored in digital memoryin a variety of encoding formats. For example, the differences (i.e.,which can also referred to as the “transition direction” or the “deltamodulation values”) can be stored as a function of pixel index.Alternatively, the cross-track positions and transition directions(i.e., the delta modulation values) of the transitions where thequantized position correction values change (i.e., the pixel indiceshaving non-zero delta modulation values) can be stored in a table suchas that shown in Table 1.

TABLE 1 Cross-track position correction function representation PixelIndex Delta Modulation Value 1357 +1 6442 −1 10204 −1 11957 −1 13318 −115042 −1

Once the position correction function 555 has been determined, the imagelines of a digital image can be modified to determine corrected imagelines responsive to the stored position correction function. In apreferred embodiment, the image lines are resampled at positionscorresponding to the pixel shifts specified in a position correctionfunction 555 such as that shown in FIG. 10C.

FIG. 11 shows an improved processing path including a print engine thatis adapted to produce printed images incorporating cross-track positioncorrections in accordance with an exemplary embodiment. The improvedprocessing path is analogous to the processing path of FIG. 4 exceptthat the resolution modification processor 420 has been replaced by aresolution/alignment processor 600, which corrects the alignmentresponsive to the position correction function 555 in addition toperforming any resolution modifications specified by the control signals415.

FIG. 12 shows additional details for the resolution/alignment processor600 and the halftone processor 425 of FIG. 11. This process is similarto that of FIG. 5 except for the addition of a position correctionoperation 610. As discussed earlier, the resolution modificationoperation 422 involves resampling the image data 350 in accordance witha resize factor. The position correction operation 610 also involves aresampling of the image data. In an exemplary embodiment, the resolutionmodification operation 422 and the position correction operation 610 canbe combined into a single unified resampling operation 620 rather thantwo sequential resampling operations.

In an exemplary embodiment, the unified resampling operation 620 uses a“nearest neighbor” resampling process where each output pixel is set tothe value of the input pixel nearest to the corresponding samplingposition. This ensures that the density of thin lines and text ismaintained. In other embodiments, an interpolation process can be usedto interpolate between the input pixel values to determine the outputpixel values at the determined sampling positions.

FIG. 13 shows an exemplary method for processing an input pixel 630 ofthe image data 350 (FIG. 12) having an associated cross-track pixelindex 635 using the unified resampling operation 620 of FIG. 12. Thisexemplary method corresponds to the special case where the resize factor417 is 2× (e.g., when the image data 350 (FIG. 12) has a resolution of600 dpi and the processed image data 428 (FIG. 12) has a resolution of1200 dpi). A determine delta modulation value step 640 is used todetermine a delta modulation value (Δ) 645 corresponding to the pixelindex 635 responsive to the pixel correction function 555. For example,the pixel index 635 can be used to look-up the delta modulation value645 in a position correction function 555 such as that shown in FIG.10B. Alternatively, the pixel index 635 can be compared to the pixelindices in a table such as that shown in Table 1 to determine whetherthe delta modulation value 645 is non-zero, and if so what its valueshould be.

An adder 650 is then used to combine the resize factor 417 and the deltamodulation value 645 to determine a repeat value 670. The repeat value670 indicates how many times the input pixel 630 should be repeated inthe line of output pixels 680. For example, If the resize factor 417 is2× and the delta modulation value 645 is Δ=0, the repeat value 670 willhave a nominal value of “2” so that the input pixel 630 will be repeatedtwice in accordance with the resize factor 417. If the delta modulationvalue 645 is Δ=−1 or Δ=+1, the repeat value 670 will be adjusted to be“1” or “3,” respectively, to correct for the cross-track positionerrors.

A repeat input pixel step 675 is then used to determine output pixels680 corresponding to the input pixel 630 by repeating the input pixel630 a number of times (e.g., 1, 2 or 3 times) according to the repeatvalue 670. The process of FIG. 13 is repeated for every input pixel 630in each image line of the image data 350 (FIG. 12). Note that eachdetermined line of output pixels 680 will be repeated twice in outputimage data given the resize factor 417 of 2×.

For the case where the resize factor 417 is 1×, a delta modulation value645 of Δ=−1 would give a repeat value 670 of “0.” A consequence of thiswould be that if the input pixel 630 corresponds to a single pixel wideline, then it would be erased from the output image. To avoid suchartifacts, if the resize factor is 1× it is generally desirable to avoidnegative delta modulation values 645. This can be generally beaccomplished by designating the color channel that is determined to havea longest cross-track line length to be the reference color channel. Inthis way, the length of the other color channels will be stretchedrather than compressed.

Even if the resize factor 417 is 2× or larger, non-zero delta modulationvalues 645 can cause the line widths of thin lines (e.g., single pixelwide lines) to be modified to a degree that a user may detect thedifference. For example, a line which would normally be two outputpixels wide after applying the 2× resize factor 417 could be one orthree output pixels wide. To avoid such artifacts, it is generallydesirable to avoid aligning the non-zero delta modulation values 645with thin features in the input image. In one embodiment, a plurality ofdifferent position correction functions 555 can be provided where thecross-track positions of the transitions are shifted to the left orright. If the user observes objectionable changes in the feature widths,then the user can select one of the alternate position correctionfunctions 555. In other embodiments, the input image can be analyzed toidentify the position of thin image features, and the positions of thetransitions can be shifted such that they are moved away from the thinimage features (e.g., into a white background region).

In some embodiments, the printer 100 (FIG. 1) includes an image capturesystem which can be used to capture images of the printed test target510 on an appropriate imaging surface as discussed earlier. In suchcases, the calibration method of FIG. 7 can be performed automaticallywithout the need for a user to manually handle the printed test target510. The calibration method can be performed at predefined intervals, orcan be initiated by a user when it is observed that the printer isproducing printed images having objectionable cross-track positionerrors.

The method for correction cross-track alignment errors that wasdescribed relative to FIGS. 7-13 can be adapted to also be used tocorrect for in-track alignment errors. FIG. 14 shows a flowchart of amethod for determining an in-track position correction function 855 thatcharacterizes and corrects the in-track position errors associated witha printhead 475 (FIG. 6A) in accordance with an exemplary embodiment.The in-track position errors may result from a variety of sourcesincluding skew of the printhead 475 relative to the imaging member 111(FIG. 2), misalignment of the individual light source chips 465 or lightsource tiles 470 within the printhead 475, misalignment of the imagingoptics (e.g., the SELFOC lens), or deformation of the imaging member111. The method includes providing digital image data for a test target800. The test target 800 preferably includes a plurality of in-trackalignment marks 870 positioned at predefined cross-track positions asillustrated in the exemplary arrangement shown in FIG. 15. The in-trackalignment marks 870 are preferably distributed along the length of theprinthead 475 which spans the test target 800 in a cross-track direction590. The test target 800 may optionally include other content such ascross-track alignment marks 570 that can be used to correct forcross-track alignment errors as has been previously described, and solidpatches 575 that can be used for other calibration or characterizationpurposes. In an exemplary arrangement, the test target 800 includesin-track alignment marks 870 for a plurality of different colorchannels. In the illustrated example, the test target includes firstcolor channel image content 580 for a first color channel printed by afirst printing subsystem 31 (FIG. 1), second color channel image content581 for a second color channel printed by a second printing subsystem 32(FIG. 1), third color channel image content 582 for a third colorchannel printed by a third printing subsystem 32 (FIG. 1), and fourthcolor channel image content 583 for a fourth color channel printed by afourth printing subsystem 34 (FIG. 1). Each of different color channelsincludes a corresponding set of in-track alignment marks 870. Thedifferent color channels can be, for example, black, cyan, magenta andyellow. However, one skilled in the art will recognize that the colorchannels can use other colorants as well. In other embodiments, ratherthan using a single test target 800 including in-track alignment marks870 for all of the color channels, they can be included in a pluralityof test targets 800 (e.g., one for each color channel).

In the illustrated example of FIG. 15, the in-track alignment marks 870are pictured as an array of equally spaced horizontal lines allpositioned at the same nominal position in the in-track direction 595.However, one skilled in the art will recognize that there are a widevariety of different alignment mark spacings and geometries that couldbe used in accordance with the present invention. In someconfigurations, the width or the in-track position of the horizontallines can be varied along the length of the line in order to enable thecentroid of the printed line to be more accurately measured. In othercases, the in-track alignment marks 870 could include crossed lines,circles, diamonds, squares or any other geometric shape that can beanalyzed to determine the in-track position of the alignment marks. Insome cases, the cross-track alignment marks 570 and the in-trackalignment marks 870 can be combined into a single set of alignment marksthat are adapted to enable the determination of both in-track andcross-track positions of the alignment marks.

In an exemplary arrangement, in-track alignment marks 870 are providedin proximity to the boundaries between adjacent light source chips 465in the printhead 475 (FIG. 6A). This reflects the fact that some of themost common sources of in-track position errors relate to positioningerrors for the light source chips 465 and light source tiles 470.Therefore, forty-four in-track alignment marks 870 can be used for aprinthead 475 that includes three light source tiles, each includingfifteen light source chips 465. In other arrangements, multiple sets ofin-track alignment marks 870 can be provided for each light source chip465. For example, two sets of in-track alignment marks 870 could beprovided for each light source chip 465, one closer to the left edge andone closer to the right edge. Preferably, at least ten in-trackalignment marks 870 are provided across the length of the printhead 475to enable the characterization and correction of localized, non-linearcross-track alignment errors.

Returning to a discussion of FIG. 14, a print test target step 805 isused to print the test target 800 to produce a printed test target 810.In a preferred embodiment, the printed test target 810 is formed on apiece of receiver 42 (FIG. 2) such as a sheet of paper. In other cases,the printed test target 810 can be an image transferred directly ontothe transport web 81 rather than onto a sheet of receiver 42. In otherembodiments, the printed test target 810 can correspond to anintermediate image formed on the surface of the imaging member 111(i.e., the photoreceptor 206) or the surface 216 of an intermediatetransfer member 112 (see FIG. 2).

A capture image step 815 is next used to capture a digital image of theprinted test target 810 using a digital image capture system to providea captured image 820. In an exemplary embodiment, the digital imagecapture system is a flatbed scanner external to the printer 100 which isused to scan the printed test target 810 formed on a receiver 42 afterit has been completely printed and fused. In other embodiments, adigital image capture system (e.g., a digital scanner system or adigital camera system) which is integrated into the printer 100 can beused to capture an image of the printed test target 810 on the receiver42 while the receiver 42 is traveling through the printer 100 (e.g.,while it is being carried on the transport web 81), or before it hasbeen transferred to the receiver 42 (e.g., on surface of the imagingmember 111 or the intermediate transfer member 112).

Next, an analyze captured image step 825 is used to automaticallyanalyze the captured image 820 to determine measured in-track alignmentmark positions 830. The measured in-track alignment mark positions 830include at least the in-track positions of the in-track alignment marks870 (FIG. 15) in the test target 800. In an exemplary embodiment, aplurality of image columns in the captured image 820 are identifiedwhich intersect the in-track alignment marks 870. The image columns areaveraged to determine a combined image trace (which can also be referredto as an in-track alignment mark profile) for each of the individualin-track alignment marks 870. Equivalently, a low-pass filter can beapplied to the image data to average the pixel values in a range ofcross-track positions, and the combined image trace can be determined bytaking a single trace through the filtered image. Preferably, any skewin the captured image 820 can be characterized (e.g., by detecting theboundaries of the solid patches 575) and accounted for in the imageanalysis process. For example, the captured image 820 can be rotated toremove the skew. Alternatively, the image traces can be taken alonglines parallel to the skew angle, or the image can be filtered using alow-pass filter which is rotated by the skew angle. The in-trackalignment mark profile then be analyzed to determine the measuredalignment mark positions 830. In an exemplary embodiment, an idealizedprofile function 528 is fit to the in-track alignment mark profile in amanner analogous to the method that was described earlier relative tothe cross-track alignment mark profile 527 in the discussion of FIG. 9B.The measured in-track alignment mark position 830 is then determined bycomputing the measure of central tendency (i.e., the centroid) of theidealized profile function 528. This approach has the advantage that itis less susceptible to noise in the image data.

Next, a determine in-track position errors step 840 is used to determinein-track position errors 845 by comparing the measured in-trackalignment mark positions 830 with corresponding reference in-trackalignment mark positions 835. In some embodiments, the referencein-track alignment mark positions 835 can correspond to ideal positionsof the alignment marks 870 corresponding to their positions in theoriginal test target 800. In some embodiments, the reference in-trackalignment mark positions 835 can correspond to the measured in-trackalignment mark position 830 for one of the alignment marks (e.g., theleftmost alignment mark or the center alignment mark). In someembodiments, one of the color channels is designated to be a referencecolor channel, and the other color channels are designated to benon-reference color channels. In this case, the reference in-trackalignment mark positions 835 for the non-reference color channels can bespecified given the known relative positions of the alignment markpositions in the original test target 800. In this way, the in-trackposition errors 845 for the non-reference color channels will reflectany channel-to-channel registration errors in addition to anywithin-channel skew.

FIG. 16A illustrates the in-track position errors 845 determined for aprinted test target 810 produced using an exemplary printhead 475. Thein-track position errors 845 were determined by computing the differencebetween the measured in-track alignment mark positions 830 and thecorresponding reference in-track alignment mark positions 835. Apositive cross-track position error 845 corresponds to the case wherethe position of the in-track alignment mark in the printed image isabove than the reference position on the printed test target 810 (i.e.,downstream relative to the printing direction assuming that the top ofthe image is printed first), and a negative in-track position error 845corresponds to the case where the printed image is below the referenceposition (i.e., upstream relative to the printing direction assumingthat the top of the image is printed first). In this example, theprinthead 475 is skewed so that the right edge of the printed image isprinted higher on the page than the left edge. Additionally, there aresome local deviations in the in-track position.

A determine in-track position correction function step 850 is then usedto determine an in-track position correction function 855 based on themeasured in-track position errors 845. The in-track position errors inthis example are scaled by the output pixel spacing so that theyrepresented in terms of the number of output pixels (e.g., the number of1200 dpi pixels). In an exemplary embodiment, a smooth function is fitto the measured in-track position errors 845 to determine an in-trackposition error function 846. For example, the in-track position errorfunction 846 can be determined by fitting a smoothing spline or apolynomial function to the measured in-track position errors 845. Suchsmoothing operations are well-known to those skilled in the art.

In an exemplary embodiment, in-track alignment corrections are appliedby resampling the image data. In this case, the resampling operationeffectively shifts the image data in the in-track direction as afunction of cross-track pixel position by an integer number of outputpixels. The required shift can be determined by quantizing the in-trackposition error function 846 to determine a quantized in-track positionerror function 847. The quantized in-track position error function 847gives an indication of how many pixels up or down the output pixelposition has been shifted. For example, the quantized in-track positionerror for cross-track pixel indices in the range of 645-2965 indicatethat the pixels are approximately one pixel below their expectedpositions.

In order to correct for the in-track position errors, an in-trackposition correction function 855 can be determined by inverting thequantized in-track position error function 847 as shown in FIG. 16B. Inan exemplary embodiment, the correction is applied by resampling theimage data at shifted pixel positions. The in-track position correctionfunction 855 gives an indication of how many output pixels the imagedata should be shifted in the in-track direction as a function ofcross-track pixel position.

A representation of the in-track position correction function 855 can bestored in a digital memory in any appropriate format to be used in thecorrection of digital image data. For example, the full in-trackposition correction function 855 can be stored in the digital memory,either in a quantized form such as that illustrated in FIG. 16B, or inan unquantized form. Alternatively, the in-track position correctionfunction 855 can be represented in other formats. For example, thequantized in-track position correction function 855 of FIG. 16B can befully represented by storing the differences between the quantizedposition correction values at sequential pixel positions. An example ofsuch an in-track position correction function representation 860 isillustrated in FIG. 16C. The in-track position correction functionrepresentation 860 can be stored in digital memory in a variety ofencoding formats. For example, the differences (i.e., which can alsoreferred to as the “transition direction” or the “delta modulationvalues”) can be stored as a function of pixel index. Alternatively, thecross-track positions and transition directions (i.e., the deltamodulation values) of the transitions where the quantized positioncorrection values change (i.e., the pixel indices having non-zero deltamodulation values) can be stored in a table such as that shown in Table2.

TABLE 2 In-track position correction function representation. PixelIndex Delta Modulation Value 645 1 2966 −1 7458 −1 7953 1 8995 −1 10971−1 11396 1 12577 −1 16854 −1 17172 −1

Once the in-track position correction function 855 has been determined,the image lines of a digital image can be modified to determinecorrected image lines responsive to the stored in-track positioncorrection function. In a preferred embodiment, the image lines areshifted in the in-track direction, where the amount of the shift variesas a function of cross-track position in accordance with the in-trackposition correction function 855.

FIG. 17 shows an improved processing path including a print engine thatis adapted to produce printed images incorporating cross-track positioncorrections in accordance with an exemplary embodiment. The improvedprocessing path is analogous to the processing path of FIG. 11 exceptthat the resolution/alignment processor 600 has been replaced by a newresolution/alignment processor 900, which corrects the alignmentresponsive to both the cross-track position correction function 555 andthe in-track position correction function 855 in addition to performingany resolution modifications specified by the control signals 415.

FIG. 18 shows additional details for the resolution/alignment processor900 and the halftone processor 425 of FIG. 17. This process is similarto that of FIG. 12 except for position correction operation 910, whichapplies both the cross-track position correction function 555 and thein-track position correction function 855. As discussed earlier, theresolution modification operation 422 involves resampling the image data350 in accordance with a resize factor. The position correctionoperation 910 also involves a resampling of the image data. In anexemplary embodiment, the resolution modification operation 422 and theposition correction operation 910 can be combined into a single unifiedresampling operation 920 rather than two sequential resamplingoperations.

In an exemplary embodiment, the unified resampling operation 920 worksby first performing the cross-track resizing and position correctionoperation using the process that was described earlier with respect toFIG. 13. An in-track resizing operation is then performed by replicatingthe processed lines to provide buffered image lines at outputresolution. An in-track position correction operation is then performedin which the output image lines are determined by resampling thebuffered image lines in accordance with the in-track position correctionfunction 855, which is preferably expressed in terms of the number ofoutput pixels that the image data should be shifted as a function ofcross-track position.

An exemplary embodiment of the in-track position correction operation isillustrated in FIG. 19, which shows an image buffer 930 containing nineimage lines, where the center image line corresponds to the nominalimage for a particular in-track position y₀. (Note that, for purposes ofillustration, the image lines in this example are shortened relative toreal image lines which can have as many as 17,000 pixels or more.) Foreach cross-track pixel index i, the image buffer 930 is sampled at ashifted in-track position y_(i) given by:y _(i) =y ₀ +C _(y)(i)  (1)where C_(y)(i) is the value of the in-track position correction function855 evaluated at the i^(th) pixel index. The shaded pixel positions inthe image buffer 930 indicate the selected pixel positions correspondingto the exemplary in-track position correction function 855. The pixelvalues at these pixel positions are copied into the output image line940. For the case where a delta modulation function is used as anin-track position correction function representation 860, the shiftedin-track position y_(i) for each cross-track pixel index can bedetermined by incrementing the shifted in-track position for theprevious cross-track pixel index by the in-track delta modulation valuefor that cross-track pixel indexy _(i+1) =y _(i)+Δ_(y)(i+1)  (2)where Δ_(y)(i) is the in-track delta modulation value for the i^(th)cross-track pixel index.

The image buffer 930 should include at least as many image lines thatare needed to cover the largest expected range of corrections for thein-track position correction function 855. After each output image line940 is processed, the image lines in the image buffer 930 are shifted upand a new image line is added to the bottom of the image buffer 930. Inan alternate embodiment, the image buffer 930, can store the entireimage. This makes it unnecessary to perform the image line shiftingoperations, but requires a much larger amount of memory which may beimpractical in many systems.

For cases where an in-line resize factor of 2× or more is used, therewill be redundant image lines in the image 930, which is an inefficientuse of the buffer memory. In such cases, it can be advantageous tointegrate the in-track resizing operation with the in-track positioncorrection operation. In an exemplary embodiment, the image buffer canbe used to store the image lines before the in-track resizing operationis performed. The image line index for each pixel position y_(i) can bedetermined as before corresponding to the output image resolution andcan be mapped to a corresponding image line ŷ_(i) in the image buffercontaining the pre-in-track resizing image lines:ŷ _(i)=Int(y _(i) /M)  (3)where M is the resize factor 417 and Int(⋅) is a function that returnsthe integer portion of a number. (Note that the same resizing factorwill typically be used in both the in-track and cross-track directions,although this is not a requirement.)

As discussed earlier, in some embodiments the in-track positioncorrection functions 855 for each color channel can be determinedrelative to a reference color channel so that they will not only correctfor the skew of the individual color channels, but will also account forcolor-to-color registration errors. In other cases, the overallcolor-to-color registration errors can be performed separately, forexample by introducing a time delay in the printing operation for thenon-reference color channels corresponding to an overall shift that isdetected between the color channels.

In some embodiments, the printer 100 (FIG. 1) includes an image capturesystem which can be used to capture images of the printed test target810 on an appropriate imaging surface as discussed earlier. In suchcases, the calibration method of FIG. 14 can be performed automaticallywithout the need for a user to manually handle the printed test target810. The calibration method can be performed at predefined intervals, orcan be initiated by a user when it is observed that the printer isproducing printed images having objectionable in-track position errors.

FIG. 20 is a high-level diagram showing the components of a system forprocessing image data according to embodiments of the present invention.The system includes a data processing system 710, a peripheral system720, a user interface system 730, and a data storage system 740. Theperipheral system 720, the user interface system 730 and the datastorage system 740 are communicatively connected to the data processingsystem 710.

The data processing system 710 includes one or more data processingdevices that implement the processes of the various embodiments of thepresent invention, including the example processes described herein. Thephrases “data processing device” or “data processor” are intended toinclude any data processing device, such as a central processing unit(“CPU”), a desktop computer, a laptop computer, a mainframe computer, apersonal digital assistant, a Blackberry™, a digital camera, cellularphone, or any other device for processing data, managing data, orhandling data, whether implemented with electrical, magnetic, optical,biological components, or otherwise. In some embodiments, the dataprocessing system 710 a plurality of data processing devices distributedthroughout various components of the printing system (e.g., thepre-processing system 305 and the print engine 370).

The data storage system 740 includes one or more processor-accessibledigital memories configured to store information, including theinformation needed to execute the processes of the various embodimentsof the present invention, including the example processes describedherein. The data storage system 740 may be a distributedprocessor-accessible memory system including multipleprocessor-accessible digital memories communicatively connected to thedata processing system 710 via a plurality of computers or devices. Onthe other hand, the data storage system 740 need not be a distributedprocessor-accessible digital memory system and, consequently, mayinclude one or more processor-accessible digital memories located withina single data processor or device.

The phrase “processor-accessible digital memory” is intended to includeany processor-accessible data storage device, whether volatile ornonvolatile, electronic, magnetic, optical, or otherwise, including butnot limited to, registers, floppy disks, hard disks, Compact Discs,DVDs, flash memories, ROMs, and RAMs.

The phrase “communicatively connected” is intended to include any typeof connection, whether wired or wireless, between devices, dataprocessors, or programs in which data may be communicated. The phrase“communicatively connected” is intended to include a connection betweendevices or programs within a single data processor, a connection betweendevices or programs located in different data processors, and aconnection between devices not located in data processors at all. Inthis regard, although the data storage system 740 is shown separatelyfrom the data processing system 710, one skilled in the art willappreciate that the data storage system 740 may be stored completely orpartially within the data processing system 710. Further in this regard,although the peripheral system 720 and the user interface system 730 areshown separately from the data processing system 710, one skilled in theart will appreciate that one or both of such systems may be storedcompletely or partially within the data processing system 710.

The peripheral system 720 may include one or more devices configured toprovide digital content records to the data processing system 710. Forexample, the peripheral system 720 may include digital still cameras,digital video cameras, cellular phones, or other data processors. Thedata processing system 710, upon receipt of digital content records froma device in the peripheral system 720, may store such digital contentrecords in the data storage system 740.

The user interface system 730 may include a mouse, a keyboard, anothercomputer, or any device or combination of devices from which data isinput to the data processing system 710. In this regard, although theperipheral system 720 is shown separately from the user interface system730, the peripheral system 720 may be included as part of the userinterface system 730.

The user interface system 730 also may include a display device, aprocessor-accessible memory, or any device or combination of devices towhich data is output by the data processing system 710. In this regard,if the user interface system 730 includes a processor-accessible memory,such memory may be part of the data storage system 740 even though theuser interface system 730 and the data storage system 740 are shownseparately in FIG. 20.

A computer program product for performing aspects of the presentinvention can include one or more non-transitory, tangible, computerreadable storage medium, for example; magnetic storage media such asmagnetic disk (such as a floppy disk) or magnetic tape; optical storagemedia such as optical disk, optical tape, or machine readable bar code;solid-state electronic storage devices such as random access memory(RAM), or read-only memory (ROM); or any other physical device or mediaemployed to store a computer program having instructions for controllingone or more computers to practice the method according to the presentinvention.

The inventive method for correcting cross-track and in-track positionerrors has been described within the context of electrophotographicprinter 100 (FIG. 1) that utilize a linear printhead having a lineararray of light sources for exposing a photoreceptor 206 (FIG. 2). Itwill be obvious to one skilled in the art that the method canequivalently be used to correct cross-track and in-track position errorsin other types of digital printers that include a linear array of lightsources, such as printers that are used to write on other types ofphotosensitive media (e.g., a printer for exposing silver halidephotographic paper). The method could similarly be used to correctin-track and cross-track position errors associated with other types oflinear printheads such as inkjet printheads that include a linear arrayof jetting nozzles for ejecting drops of ink onto a receiver media.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations, combinations, and modifications can be effected by a personof ordinary skill in the art within the spirit and scope of theinvention.

PARTS LIST

-   31 printing subsystem-   32 printing subsystem-   33 printing subsystem-   34 printing subsystem-   35 printing subsystem-   38 print image-   39 fused image-   40 supply unit-   42 receiver-   42 a receiver-   42 b receiver-   50 transfer subsystem-   60 fuser module-   62 fusing roller-   64 pressure roller-   66 fusing nip-   68 release fluid application substation-   69 output tray-   70 finisher-   81 transport web-   86 cleaning station-   99 logic and control unit (LCU)-   100 printer-   111 imaging member-   112 intermediate transfer member-   113 transfer backup member-   201 first transfer nip-   202 second transfer nip-   206 photoreceptor-   210 charging subsystem-   211 meter-   212 meter-   213 grid-   216 surface-   220 exposure subsystem-   225 development station-   226 toning shell-   227 magnetic core-   240 power source-   300 page description file-   305 pre-processing system-   310 digital front end (DFE)-   315 raster image processor (RIP)-   320 color transform processor-   325 compression processor-   330 image processing module-   335 decompression processor-   340 halftone processor-   345 image enhancement processor-   350 image data-   360 metadata-   370 print engine-   400 print engine-   405 data interface-   410 metadata interpreter-   415 control signals-   416 resolution modification flag-   417 resize factor-   418 halftoning flag-   419 halftoning parameters-   420 resolution modification processor-   421 modify resolution test-   422 resolution modification operation-   425 halftone processor-   426 halftone image test-   427 halftoning operation-   428 processed image data-   430 printer module controller-   435 printer module-   450 printed image-   460 light source-   465 light source chip-   466 connection pad-   470 light source tile-   475 printhead-   500 test target-   505 print test target step-   510 printed test target-   515 capture image step-   520 captured image-   525 analyze captured image step-   526 combined image trace-   527 alignment mark profile-   528 idealized profile function-   530 measured alignment mark positions-   535 reference alignment mark positions-   540 determine cross-track position errors step-   545 cross-track position errors-   546 cross-track position error function-   547 quantized cross-track position error function-   550 determine position correction function step-   555 position correction function-   560 position correction function representation-   570 alignment marks-   575 solid patches-   580 first color channel image content-   581 second color channel image content-   582 third color channel image content-   583 fourth color channel image content-   590 cross-track direction-   595 in-track direction-   600 resolution/alignment processor-   610 position correction operation-   620 unified resampling operation-   630 input pixel-   635 pixel index-   640 determine delta modulation step-   645 delta modulation value-   650 adder-   670 repeat value-   675 repeat input pixel step-   680 output pixels-   710 data processing system-   720 peripheral system-   730 user interface system-   740 data storage system-   800 test target-   805 print test target step-   810 printed test target-   815 capture image step-   820 captured image-   825 analyze captured image step-   830 measured in-track alignment mark positions-   835 reference in-track alignment mark positions-   840 determine in-track position errors step-   845 in-track position errors-   846 in-track position error function-   847 quantized in-track position error function-   850 determine in-track position correction function step-   855 in-track position correction function-   860 in-track position correction function representation-   870 in-track alignment marks-   900 resolution/alignment processor-   910 position correction operation-   920 unified resampling operation-   930 image buffer-   940 output image line

The invention claimed is:
 1. A digital printing system incorporatingin-track position corrections, comprising: one or more printingsubsystems, each printing subsystem including a linear printheadextending in a cross-track direction including an array of light sourcesfor exposing a photosensitive medium; a data processing system; adigital memory for storing an in-track position correction function; anda program memory communicatively connected to the data processing systemand storing instructions configured to cause the data processing systemto implement a method for determining an in-track position correctionfunction for at least one printing subsystem, wherein the methodincludes: a) providing digital image data for a test target including aplurality of alignment marks positioned at predefined cross-trackpositions; b) printing the test target using the digital printing systemto provide a printed test target; c) using a digital image capturesystem to capture an image of the printed test target; d) automaticallyanalyzing the captured image to determine a measured in-track positionfor each of the alignment marks; e) comparing the measured in-trackpositions for the alignment marks to reference in-track positions todetermine measured in-track position errors; f) determining the in-trackposition correction function responsive to the measured in-trackposition errors, wherein the in-track position correction functionspecifies in-track position corrections to be applied as a function ofcross-track position; and g) storing a representation of the in-trackposition correction function in the digital memory; wherein the digitalprinting system is adapted to print digital images using a printingprocess that includes: i) receiving digital image data for a digitalimage to be printed by the digital imaging system, wherein the digitalimage includes a plurality of image lines extending in the cross-trackdirection; ii) determining corrected image lines by resampling thedigital image data responsive to the stored representation of thein-track position correction function; and iii) printing the correctedimage lines using the one or more printing subsystems to provide aprinted image with reduced in-track position errors; wherein the linearprinthead includes a plurality of individual light source chips, eachincluding a plurality of light sources, and wherein alignment marks arepositioned in proximity to boundaries between adjacent light sourcechips; and wherein the representation of the in-track positioncorrection function stored in the digital memory includes thecross-track positions and transition direction of transitions in thein-track position correction function.
 2. A digital printing systemincorporating in-track position corrections, comprising: one or moreprinting subsystems, each printing subsystem including a linearprinthead extending in a cross-track direction including an array oflight sources for exposing a photosensitive medium; a data processingsystem; a digital memory for storing an in-track position correctionfunction; and a program memory communicatively connected to the dataprocessing system and storing instructions configured to cause the dataprocessing system to implement a method for determining an in-trackposition correction function for at least one printing subsystem,wherein the method includes: a) providing digital image data for a testtarget including a plurality of alignment marks positioned at predefinedcross-track positions; b) printing the test target using the digitalprinting system to provide a printed test target; c) using a digitalimage capture system to capture an image of the printed test target; d)automatically analyzing the captured image to determine a measuredin-track position for each of the alignment marks; e) comparing themeasured in-track positions for the alignment marks to referencein-track positions to determine measured in-track position errors; f)determining the in-track position correction function responsive to themeasured in-track position errors, wherein the in-track positioncorrection function specifies in-track position corrections to beapplied as a function of cross-track position; and g) storing arepresentation of the in-track position correction function in thedigital memory; wherein the digital printing system is adapted to printdigital images using a printing process that includes: i) receivingdigital image data for a digital image to be printed by the digitalimaging system, wherein the digital image includes a plurality of imagelines extending in the cross-track direction; ii) determining correctedimage lines by resampling the digital image data responsive to thestored representation of the in-track position correction function; andiii) printing the corrected image lines using the one or more printingsubsystems to provide a printed image with reduced in-track positionerrors; wherein the in-track position correction function is quantizedto integer in-track position corrections; and wherein the representationof the in-track position correction function stored in the digitalmemory includes the cross-track positions and transition direction oftransitions in the quantized in-track position correction function. 3.The digital printing system of claim 2, wherein the digital printingsystem includes a plurality of printing subsystems for printing acorresponding plurality of color channels, wherein one of the colorchannels is designated to be a reference color channel and the othercolor channels are designated to be non-reference color channels, andwherein the reference positions for the alignment marks for thenon-reference color channels are determined responsive to the measuredpositions of one or more alignment marks printed with the referencecolor channel.
 4. The digital printing system of claim 3, wherein apredefined color channel is designated to be the reference colorchannel.
 5. The digital printing system of claim 2, wherein thereference positions correspond to ideal positions of the alignmentmarks.
 6. The digital printing system of claim 2, wherein the printedtest target is on a print medium, and wherein the digital image capturesystem captures an image of the printed test target on the print medium.7. The digital printing system of claim 2, wherein the printed testtarget is on an imaging surface, the imaging surface being a surface ofa photoconductor, a surface of an intermediate transfer member or asurface of a transport web, and wherein the digital image capture systemcaptures an image of the printed test target on the imaging surface. 8.The digital printing system of claim 2, wherein the plurality ofalignment marks includes at least ten alignment marks.
 9. The digitalprinting system of claim 2, wherein the light sources of the linearprinthead are LED light sources.
 10. The digital printing system ofclaim 2, wherein the resampling of the digital image data is alsoresponsive to a resize factor.
 11. The digital printing system of claim2, wherein the digital image capture system is a component of thedigital printing system.